You gotta go steady with them mate. It's a tough one. I've worked mine up gradually over the months. Some would say running 7.5 mile is too much for a relatively young pup.
5 mile sounds a lot for 13 weeks. It could be steady mooching with lots of stopping though which is a different ball game to 5 mile flat out by the bike.
Try mental stimulation aswell as physical. Get your pup retrieving from water. It is good exercise and uses his/her head a bit. Throw a tennis ball into some long cover and get it using it's nose to find it etc.
It's hard to knacker them out, without knackering