One of the big challenges I have at the moment, is that all my focus is on my Ironman training. This means I have very little time to do any resistance training. I have had to get creative. I put a pull-up bar in the doorway that leads from my bedroom to the lounge-kitchen-dining room area.

In the evenings or mornings between cooking or cleaning or doing whatever other things, I'll quickly do 10 pull-ups, then go back to whatever I was doing, then a few minutes later, I'll go and quickly do another 10 (it only takes 30 seconds). Then a few minutes later if I have to walk into the bedroom, passing underneath the pull-up bar, I'll bang out another 10.

I'll follow this pattern for the duration of whatever I'm doing. The total time of actually exercising might only be 4 minutes, but I have been able to crank out 50-60 pull ups without putting much of a dent in my schedule.

I'll do that with pull-ups maybe twice a week. I do the same with push-ups and some ab work too.
For the ab work I'll use the pull-up bar too, just grab it and do some hanging leg raises. I'll also set an exercise mat up in the lounge, to do planks and a couple of plank variants. I'll alternate between the planks and the hanging leg-raises.

For the push ups I'll do 20 regular ones for my first set as a 'warm-up', and then do decline push-ups, putting my feet up on the arm of the couch (don't tell the missus).

Again, total exercise time might only be 4-5 minutes, hardly making a dent in your schedule, but you have been able to get in a solid ab workout or crank out 60-100 push ups.

Its not a perfect workout, I realise that, but it still allows me to keep my muscles strong so that I dont lose too much.

When this Ironman thing is over, switching back to HIIT and more serious weight training will not be as difficult, because I have been able to maintain myself in this way.

You really can fit it in if you are willing to get a little creative, sure it won't always be perfect, but something is always better than nothing.