How much time do you have to train, and how many days/week are you able to train?
You sound like a relative beginner, so you can expect pretty rapid improvements from just riding more. Frequency beats duration at this point. Try to get out as often as possible, and ride as long as you have time for.
When you feel yourself stagnating (which will probably happen pretty fast, with the low intensity), or if you already have a decent level of fitness, it is time to include some intensity.
As a basic rule, the fewer hours each week you have to ride your bike, the harder those hours should be. You should focus on raising your lactate threshold (the maximal point you can ride at for prolonged periods) as much as possible. If you have a long hill nearby to train on I would highly recommend using that. Try to ride for 15-30minute periods at a HARD pace. How hard? Hard enough that, at the end of the interval, you'll doubt whether or not you will be able to finish the last couple of minutes - a little bit easier than this is OK to.
You can include some longer intervals as well at a slightly lower intensity, where you can complete about 45-90 minutes without struggling too much, but it should require some mental focus to maintain the pace.
Make sure to have enough recovery during the week. I would recommend at least two days off (as a minimum). A schedule of riding every other day will probably work great for you, and will insure you are adequately recovered before each workout.
As your fitness increases you can include more frequent rides and structure your week in blocks, in order to accumulate more overall stress. A block could be Tuesday, Wednesday, Thursday. and then Saturday, Sunday.
for now though, a sample plan could be something like this:
- Monday: 2x15 minutes at a HARD pace with about 5-10minutes of
recovery in between.
- Wednesday: 2x20 minutes at a pace slighty easier than monday.
- Friday: 90 minutes a a pace slightly easier than thursday.
- Sunday: A long ride, or just restart the cycle by doing the same
workout as you did Monday.
Now, that is pretty basic, and you will probably progress pretty fast on this plan. You can change it up a little bit, like adding an extra interval once in a while, making it harder or making the interval longer.
You can even try riding consecutive days in a row.