I'm 57. Only climbing/working 2/3 days a week now. On the odd occasion that I do a full week, I'm shattered. I don't know how fit I am, but I've a body used to it.
Also consider, it will be a while before you establish yourself and start making profit. If you're considering setting up on your own, the initial outlay on equipment is very expensive.
Maybe test the water first. Plenty of companies looking for groundies.
Unless you have a particular passion for trees/ Tree work, I think in your position, I'd be looking to retrain in something with a bit more longevity.
At best, you'd be lucky to get 8-10 years, before you're knackered.