It sounds like you're off to a great start with minimalist shoes. The first rule is to start slow and increase usage gradually, and you have done so.
Make sure the shoes fit perfectly. You can get blisters and pain in minimalist shoes pretty easily, especially on long runs, if they don't fit perfectly or you're not wearing them correctly. Also, if the shoes don't fit snugly and conform to your feet, you will miss a lot of feedback and your proprioception will suffer, thus increasing the chance of pain or injury.
My first pair of Vibrams were too large and I would constantly get blisters due to some parts of the shoe chafing the skin. I had to almost always wear toe socks with them. The second year I made sure the shoes fit better, and since then they have been more comfortable, and I no longer wear socks with them. I still occasionally get some sore spots around the heel or side of the foot on long runs.
Focus on running form
Never be complacent with your running form. Constantly monitor your form and do some reading. It isn't just "run on the balls of your feet" (actually, my heels touch the ground lightly near the end of my step), there are a lot of subtle details about your form that can continually be improved. You don't get perfect form and prevent injury just by putting on minimalist shoes or going barefoot, just like traditional running shoes don't by definition cause injury. Minimalist shoes do make it easier to learn and maintain good form because they don't mask the impact of bad form as much as a cushion shoe.
I started running completely barefoot this year and read the book Barefoot Running Step By Step. I realized that while Vibrams do give you a lot more feedback than cushion shoes, and thus help encourage better form, there's still a significant difference between that and going barefoot. In retrospect, I've realized that I was being too be hard on my feet and joints in minimalist shoes. In previous years, my feet and joints would ache after a long run in minimalist shoes, but this year, after some significant improvements to my form, I feel tired but refreshed after a long run in minimalist shoes, with no unnecessary aches or pains.
At this point, since you've done a good transition in to minimal shoes and are already logging 5 mile runs, if you aren't just tired but hurt after a long run in minimalist shoes, then there's likely something you can improve with your form. Yes, the transition to minimalist shoes can involve some muscle and foot soreness as your body adapts, but don't be a hero and push through pain.
From the Vibram FAQ:
We recommend running first completely barefoot on a hard flat surface.
In hindsight, I think this is key. Running barefoot on a road (and even on gravel) will maximize the sensory input to your feet. You'll feel every nuance of your form:
- What part of your foot lands first and last
- How softly your are making contact with the ground
- Whether your are twisting or scuffing your feet (you shouldn't)
- Whether your are pushing off the ground rather than simply lifting your feet
- Whether your feet and legs are relaxed (thus able to handle a rough surface) or stiff
- Whether your cadence is quick enough (faster cadence = lighter step)
- Whether your knees are sufficiently bent (improves the spring action of your feet and legs, allowing a lighter step, forefoot strike, and activation of the calf)
You don't need to "go easy" by sticking to softer surfaces, whether barefoot or in minimal shoes, and by doing so you are likely to miss a lot of key lessons about proper form as the surface is more forgiving.
Just a few runs on pavement or gravel can really fine-tune your form and teach you how to run efficiently and avoid injury. These lessons will apply to minimalist shoes and cushion shoes too. I like to do a few miles barefoot at the beginning of a long run to fine-tune my form, on a variety of surfaces including asphalt or gravel, and this carries over nicely to the main part of the run performed in Vibrams.
I'm not an expert about this, but I think focusing on form could help a lot as you spend more item in minimalist shoes. I developed knee pain earlier this year due I'm pretty sure to overpronation and a tight hip flexor, even while wearing minimalist shoes. By focusing on form I've nearly eliminated the problem.
Occasional barefoot running has taught me to increase my cadence a little more, correctly align my hips and feet, land more lightly, and run downhill properly—even while wearing minimalist shoes or traditional shoes. This has nearly eliminated my pronation and knee problems.
Regarding sore feet
I am limited in mileage because the bottom-center of my foot gets gets tight/exhausted
A few points of advice:
- Your feet may still be adjusting to the new biomechanics. You may need to just take your time increasing your distance so that your feet can continue to strengthen
- You may need to lighten your step. In my first two years running in minimalist shoes, I would feel the same tightness and fatigue in the center of my sole. Since then I have learned to run lighter on my feet, with even higher cadence (100+ strides per minute rather than 90), and I don't really have this problem anymore. If you use a smartphone GPS app to track your runs, note that some of them (such as iSmoothRun) will track your cadence
Again, don't overdo it. If after some time you can't seem to get past a certain number of miles in minimalist shoes without this problem, then you may need to seek a more experienced opinion than mine.
Also, if the top of your foot gets sore, then you may need to tweak the way you wear and lace your shoe, or possibly consider a different type or size of minimalist shoe. Because they fit small and snug, there can be a lot of pressure on the top of the foot from the laces or straps.
I think you can continue to gradually increase the distance you run in your minimalist shoes. I was able to complete 12 mile runs in my first year with no problem, and 16 miles the following year, and it sounds like you're in better shape than I am (I've never attempted a marathon for example). The “transitioning tips” bullet points at the end of the Vibram FAQ sound like good guidelines.
There should be no problem with wearing other shoes for some portion of your runs until you get to the point that you're ready and willing to use minimalist shoes or go barefoot 100% of the time. In the first two years I used traditional running shoes on roads, and I still use heavier shoes during the winter months.
When you run in your other shoes, try to imagine that you're in minimalist shoes and apply the form you use there in your other shoes. This can only work to a certain degree of course, but that could help you maintain your form. You may find that you will be running better in both types of shoes.
I started running with minimalist shoes 2 1/2 years ago, and by the end of that year I was doing 90% of my runs in Vibrams (excluding winter runs in snow). This year, I added true barefoot running to the mix, doing that about 30% of the time now. Before getting into minimalist shoes, I wore a variety of cushion shoes from the basic road shoes to some pretty heavy duty trail shoes.