100 miles of commute plus a 3-hour ride on a Sunday is enough to give you opportunities to get seriously quick. Tadawdy's suggestions are excellent. When I was commuting, I used to pick two or three journeys each week - say, Monday, Wednesday, Friday evenings - and try to ride at threshold, traffic permitting, the whole way. Threshold being the highest level of effort you could sustain for an hour, but you don't need to get scientific about it, you can do it by feel. On the Sunday ride, why avoid the hills? If you could incorporate a few hills into your lap, and attack them reasonably hard, that would introduce some intensity into that ride and you'd soon see the difference. Alternatively, just put some intervals into your middle lap - say 6x60seconds sprinting as hard as you can, with 2 minutes recovery between each.

Yes there is a point of diminishing returns. Lots of miles is good, but simply churning out distance at low intensity will take you only so far. Having said that, keep the intensity reasonably brief. The coaches say that the most common mistake is to make the hard rides too easy, and the easy rides too hard. You need variety, to be riding within yourself for the bulk of the time, but absolutely going for it for short periods.