Originally Posted by JonathanGennick

Your friends are just making polite conversation with you. They mean "I wish I could" in a very abstract way. When my cousin tells me of his latest trip to the Boundary Waters, or to Isle Royale, I probably respond very similarly sometimes. For example, I might respond by saying: "Yeah, that'd be a great to do someday", or "I'd love to do that someday." It's a way of acknowledging how cool something is that someone else does. And I sort of mean what I say, in that I do think the things my cousin does are cool, and I do think they would be fun to do. But, truth be told, I have other things that I am interested in more than canoeing and backpacking (like biking and cross-country skiing).

So your friends are just saying in their own way that they think what you do is really cool. Just take that for what it is, and don't put them on the spot by trying to get them on a bike. The few who are really serious about bike commuting will follow-up more purposefully. You'll know when someone is serious.