I'm going to keep this as simple as possible. Hopefully some of these answers will help others in similar situations.
- Injury: If you have a injury then you should ask ONLY your doctor what you can do and they can advice you on specific sporting activities. There is no point asking here on SE if you can run/walk/cycle/swim/juggle with your dodgy knee/foot/hip/lungs/balls.
- Fitness you have previously done: Unless you have been active in doing your activity in the last 6 weeks, assume you have lost a significant amount of fitness level over that time compared to your previous peak. It is wrong to assume that because you ran a marathon when you were 23 that now you are 48 you can do a few weeks/months training and run a ultra-marathon.
Example: If my barbell squat working weight is 120kg and I take a 6
week break from squatting; then I expect to start at a lower working weight
(e.g 90kg) when re-starting regular exercise. I will have to slowly work back up to my previous weight and the time that takes is variable on a lot of personal factors.
- Yoga gives transferable fitness: here is a controversial "opinion": Yoga ONLY makes you good at similar yoga-based activities, eg. muscle stabilization (balance) and mindfulness (There are some others but the studies are a bit wishy-washy). Yoga does not mean you are able to run a mile/5k/10k/marathon in a good time. In the same way that me being able to lift X weight does not mean I can easily swim a mile. *Note: This doesn't hold true for all sports; a person with a sub 20 minute 5k time will probably be able to cycle 20k in a pretty good time. Use your brain.*
- Injury, that isn't: Only by doing exercise, regularly, consistently, and most importantly: listening to your body and PUSHING YOUSELF can you make any progress at anything. You need to go do some exercise and learn how to listen to your body to understand how far you can push yourself. Don't focus on the micro and lose sight of what you must do to achieve your goals.
Example: Just because I overdid it on squats 2 days ago and have DOMS
doesn't mean i'm skipping my 10k run today, I just plan to do a longer
warm-up and expect to dig a little deeper mentally.
"Everybody wants to be a bodybuilder, but nobody wants to lift no heavy ass weights." Ronnie Coleman, Ronnie Coleman: The Unbelievable!
- Gender/Weight is the reason I can't run/jog/walk: If you can't run, jog. If you can't jog, walk. If you can't walk then stop until you can walk again.
Fitness is all about YOU, you are aiming to beat only yourself and trying to hold yourself up to the standard of others only will result in disappointment. We can't all be the fastest/strongest but we can be faster/stronger than our past selves.
Specifically, long distance running has a huge mental-only aspect to it. Having the mental grit/determination to carry on after the 10th mile requires a lot. You need to develop your "don't quit" attitude through practicing doing long-distance runs (10k/Halfs). This determination is highly personalised and what gets your through "the wall" depends on your goals and motivation.
Check with your doctor about your hip/knee issues.
If you want to train for a marathon, go out and RUN.