The biggest challenge in intervals for me is the recovery period. I just hate to ride at the speeds necessary for me to manage zone 1 & 2. So I really focus on hitting the 5b/5c zones and do recovery in zone 3/4. Seems to work Ok. I would stress though that doing intervals is not as good as riding fast group rides in my experience, theoretically it should do the same but for me the groups rides bring out the extra effort that I cannot seem to get at just doing intervals by HR.