I live 15 miles from work and have started commuting. I love starting and ending my day on the bike.

The commute is either a MUT or straight and flat country roads. I’m wondering how to incorporate intervals and training into my commute. I’ve tended to ride very easy most days (zone 2 or 3) and do one or two prolonged intervals one or two days a week. On the weekend I do a big ride (3 or 4 hours) or not quite as big ride with intervals (normally 3x20) on the weekend.

Any suggestions?

Commuting 15 miles is long enough for a good workout. When I had an 11-mile commute, I'd ride easy on the way into work, and then fairly hard on the way home. There's a "sweet spot" that's just hard enough to be sustainable, about like 40k-tt intensity, minus a bit.

If you can go "the long way" home some days and make it 20-25 miles, that'll give you 45 minutes-hour of "sweet spot" training after warming up. That'll do you loads o' good.

I'm going to assume these flat country are relatively stop-free...

I'm glad to hear it. I rode in this "sweet spot" last night on my fixie (48x16) and it was 35 minutes. The roads are part of a monster grid of near-empty country roads. I'll probably think about adding a few more miles.