I've been doing 30-minute sessions on the bike spiking it to 175 and back down to 140. I find it is better than grinding out a high plateau. I think the technical term is HIIT. It's like doing sprints.

The GPS is great, especially when doing random courses. I do have a nice running track nearby (GPS is pointless for a 400m oval, unless I forget how many laps I did). Maybe I'll hit it up in the fall a few times. There are lots of mountains near me and I find the ones that are about 500m up are a decent workout. Again, fall.