Turning 46 in a couple of months, and at my fastest and fitest both on the mtb and road bike in very long time.
One of the main things that helped this is ebikes, I have one, I ride it up trails* and its like doing intervals, but technical and fun. I also had chainsaws and digging tools on my back which made it interesting.
And embrace age, enjoy your surroundings, road biking I love for this, my avg speeds are all over the place, but I don’t care…
If you’re not there in your head, you wont be in body
*Don’t worry ebike haters these are trails we have opened and dont see any other traffic…