Generally speaking, you'll get good at whatever you specifically train for.
If you want to get generally more bike fit, just get out and ride. If you want to work on your speed, practice intervals of faster pace, if you want to work on your climbing, do hill reps. If you want to be just generally fit then mix it up with some hiking/running/swimming/other sport - that's probably the all-round healthiest option.
Easy answer, as long as you're enjoying it and getting out on the bike regularly you'll naturally build up your speed and the length of ride you can easily manage, no real need to worry about training plans unless you really want to optimise your potential.
Generally speaking I spend more time trying to work on my technique (cornering, descending, smooth pedalling, cadence, and since I got a mountain bike a whole heap of off-road stuff to think about) than I do on fitness, which seems to look after itself.