[Closed] Help me get fitter! HIIT?

ok, my time for training is limited because of work and kids. I want to make the most of my commuting time to help me get fitter for mtb, fell running, road running and duathlons.

I've get a few races planned but nothing longer than about 10 miles (running) and 30miles (mtb "challenge"!) for the moment. Current 10k PB is 37:30.

I commute for about 25miles each way. It's made up of one of:
- Ride door-to-door; ride home the next day
- Run to one of 4 stations (2miles, 5miles, 7miles or 6miles hilly) and get the train
- Ride to any of those stations (road, canal or hilly)

At the moment I exercise on 4-6 of the 10 commutes in the week but don't really have a structure/ method to maximise the benefits. I don't have a HRM.

Presumably HIIT would help, but it this just a few reps of thrashing and recovery built into one or more of the runs/ rides I'm doing anyway?

Any suggestions welcome!