It sounds like you want to do two things, which are not necessarily connected but aren't contradictory either:
1. get faster, as measured by your time on your commute and/or 10m TT
2. get better endurance, so you can do longer rides without killing yourself
Getting faster in this sense isn't really a sprint, more of a lactate-threshold type workout. So doing intervals will help, as will time trials. But remember not to ride every commute as a time trial! You need to chill out on your "easy" days and ride hard on your hard days. Otherwise you wear yourself out so you never go hard enough on the hard days.
As gonzo says, training for endurance requires you to ride long rides at well below your threshold. Any long social ride, assuming it isn't with a psycho cycling club, should help - but to make sure you don't go too fast or too slow, a large amount of it could/should be solo. Riding on your own is nice, in a zen kind of way.
Improving your TT will also improve your endurance, to some extent, in that it will increase your lactate threshold and thus make you work at a lower proportion of your maximum on the long rides. But if you want to be good at riding long distances ... you have to ride long distances.