IMO the reason fitness efforts fail is that for one reason or another people stop trying. Expecting too much, too soon is a great way to lose interest. I suggest for the first 6 months to get off the bike feeling like you could have done more. If you want to do multiple "sets" of activity I'd recomend walking. I think the real sucess isn't how many miles done or how fast you are, but that you're doing something. This doesn't have to be work. Eventually putting 100% effort into a workout will be fun. Give yourself the gift of time to get there. Think years not weeks.