Do you think my nearly 4 year old, recently started 15.2 horse could manage more than an hour's hack without his back or muscles being pushed too hard? He's been hacking out for 2 weeks, & is out 24/7 & is very active in the field. I am stuck with tarmac until I can take him out for an hour & a half minimum to get him onto some quiet lanes. I don't want to put him off being ridden by tiring him out or giving him a sore back. He has a comfy sheepskin lined treeless endurance saddle, & is very forward-going, but this is a mental rather physical thing.