You have my sympathies and my respect in equal measure my friend.
Speed is less of a relevant measure than heart rate to be honest and this would be a good time to do some work outs based around that.
If it helps, my max HR on the turbo seems to be 190 and I tend to try and average 166 over a 60-70 minute session, where the ramp up is from about 140 onwards (so to get to an average of 166, I tend to ramp up and spend quite a lot of time at 160+)
Once you got some bench mark data under your belt you’ll start to learn what you can do. Once you know you can do it on the turbo then when you get back on the road or mountain bike, if you’re hurting at say less than 85% of your max, you’ll know that whatver your body is saying your heart still has more to give. It’s quite a rewarding experience really.