Training tips based on HRT data and race results?

Hi all, I did a 60 mile MTB XC race this summer, got through on 4.14 averaging 89% of max heart rate. Seems like I put a decent effort in even if the time isn't much to brag about. If I'd like to squeeze by in less than 4 hours, is it possible to say anything sensible about how to train to achieve that? My main training dose is from commuting. It's a 15 mile ride to work, and it takes me about 1.03 to 1.10. There's a shower and a dressing room at work so I have no problems hitting it hard. A gym too if need be. I can get about 10 miles of reasonably open stretches before I hit inner city traffic and have to give focus to that. I've tried sneaking in some interval training by honking up the 3-4 climbs worth mentioning along the route, but I can't say I've noticed any particular effect. I'm male, age 43, just shy of 6', and 80 kg. I also do a fair amount of skating, and used to be a masher, so leg strength is probably not an issue.