I get asked this question occasionally - I don't know why. It's not like I look like a victim of starvation, and weight counts for a lot in cycling.
I had to think about this. I don't have a training program. I don't do intervals or attempt to estimate my threshold power or bother with a power meter. I just ride. I'll hammer it occasionally, but not as a deliberate activity. If I hammer it, it's because I feel like burning off a bit of steam. I'm always conscious of the fact that because I commute, I need to get home in the evening, and I need to get out of bed before dawn and make it into work the following day. I don't want to totally cook myself and then find that the bus is the only option because my legs refuse to rotate.
It was different when I was a teenager and training for school sports. Our coaches would deliberately hammer us into the ground so that our muscles would rebuild stronger and stronger after each session. The same went for when I was doing gym work in my 20's - the theory (and practice) was all about overtaxing a muscle group, which would then force it to repair and strengthen. I found gym work pretty easy - I'd do a different muscle group each day, giving the one I had beaten to death the day before some time to recover before it was hammered again. In that fashion, I bulked up pretty quickly. So much so, I walked like a gorilla for some years. It was so easy - train each muscle group to failure, eat lots of protein and get plenty of rest.
I had some bad experiences in my first few years of riding to work - they generally revolved around killing myself on the way in and then suffering really badly on the way home. I'm not in this for the pain, so I feel no urge to smash myself into the ground every other day.
The interesting thing is that since November, I've been riding a hell of lot harder than normal. Harder than ever in fact. And instead of being a gibbering mess, I'm actually feeling better than I can ever remember. I must have hardened up somewhat. I guess the lesson is that there is no point in going at it soft.