To be honest, I sometimes feel he's better when he's had less exercise!
This could very well be the case. If you walk a dog for hours and hours, all you get is a super fit dog who is impossible to tire out.
What do you to stimulate his brain? Does he get chew toys, puzzle feeders, stuffed kongs and scent games to keep him occupied?
I'm a dog walker and every single working spaniel I walk (cockers, sprockers and springers) ALL have some kind of "job" to do. One does water retrieving, some do cani-cross, some do agility - but they all do something outside of normal walks.
At eight months he may be too young for cani-cross or agility but you could certainly do scent games with him - so take a ball on walks, hide it in long grass and get him to find it, scatter his kibble in the grass and get him to sniff it out. Ditch the bowl and feed him all his food via puzzle feeders, kongs or training games.
On walks, practise sit/stays, practise recall, teach him tricks - get him to sit and ask him to go around objects to the left/the right, teach him to jump over a log or go under a branch etc. Work his brain, not just his muscles.
I have a working breed (beagle, not cocker) - he's three and he does best on 60-90 minutes exercise a day. Anything more and he gets over-stimulated and silly. Any less, and the same applies. 60-90 minutes appears to his "golden" amount where it tires him out but doesn't make him giddy 
He also gets a nice, low-fat natural chew everyday after walks, which helps keep him busy and it calms him down as well.
Best of luck! Remember, you're bang in the middle of the teenage years and it's really tough at this age - it DOES get better. 