Ride gently- gradually increasing the miles and the effort. Try to ride often but Not every day. Give a bit of time for the legs to adjust to the "Extra" strain you are going to put on them.
Doesn't matter what bike you have right now- providing it works and will not break down. In a few months time though- you may feel it does so start saving.
They say 30 minutes exercise 4 times a week over here to Keep the fitness you have. Milage doesn't matter but as you will find out- Milage will go up as you re-find your fitness.
So get riding and enjoy yourself.
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How long was I in the army? Five foot seven.
Spike Milligan