Adjusting Training Plan for Outside Rides

I’m on a low volume training plan as I have an auto immune disease and don’t recover well. I currently have one vo2 (Tue) , one sweet spot (Thur) and one threshold (Sat) workout per week. This volume works well for me but I am missing riding outside. If i do the three days indoor I’m a bit tired after an hour or so of my outside ride on the Sunday. If i want to do a decent outside ride on Sunday I’d have to drop the Saturday threshold ride but then I would never progress up the levels as TR doesn’t factor in outside rides into progression levels. Any suggestions on how I get around this?

You could associate an outdoors ride with an outdoors workout and it’ll gain the PLs from that. You’ll have to be quite careful about what you match though. You don’t want to be matching, a 99% relaxed 2 outdoors ride finishing with a 30s sprint, with VO2max etc.