I really just do the intervals and easy recover between them. During the brief warmup, I usually do a couple jumps at full interval intensity for only 20% of the planned duration (so, 1m at 400W for my 5m intervals, or 5-10s at ~800W for my 1m intervals).
I think that after your intervals, if you have time, an endurance ride would be fine, maybe working up to SST if it started to feel right. I don't think that's as valuable as the intervals though. It'll help build aerobic base if you don't have several years of miles in your legs. With my history, I don't need the extra volume to compete in 2.5-3.0 hour road races. I tend to get a couple 60 mile training rides in each month, and that's plenty of endurance work for me.