if its nothing medical, then maybe hes just unfit or doesn't like running so much, just like people are different so are animals, praise him and try a combination of running and walking or a stop and play halfway, dragging him is likely to make running punishment rather than a fun, bonding exercise.

Maybe you should get the vet to look Blake over, as there might be a reason he is reluctant to go far. It is best to rule out anything and safer for him. It is the best thing to do first rather than push the issue on uncertainty.