That's great you're on a bike at age 40! It's really good exercise for mid-life -- low impact and reasonably easy on the joints ...

As for building leg strength, I confess I don't really have any well-informed advice. My general sense is that it's just a matter of cycling regularly .... the more you do it the easier it gets, even if it takes a really long time. Several folks have suggested I do some strength training too, weightlifting in the gym, and I'm sure that would be good for me but I'm too lazy to go!