Hi! Ours has done 25 miles although he‘s show line and, for a cocker, isn‘t wildly energetic. There was a thread on here a couple of years back from someone who did a week long hike with his cocker to raise money for a spaniel charity. As long as they‘re fit, have ample opportunity to stop and rest now and again and have enough to drink I don’t foresee a problem. My dog doesn‘t like the heat much either so obviously I wouldn‘t do it on a hot summer day.
With regard to the lead, I actually find it more tiring for me and Humphrey as he‘s used to being off lead most if the time anyway, I found that while he races back and forth for the first few miles, he starts to pace himself at some point automatically.
He‘ll probably zonk out in the evening
but provided he‘s fit, it won‘t do him any harm.
Enjoy your walk!