depends on the dog ........what it's use to , how strong it is and how well it moves ....... some pups/dogs trot like they're floating , some move horribly rough/incorrectly ........ i'd put the dog on a long lead so i had some control over what he does and let him show me what he wants to do .... i'd walk him , let get into a trot if he wanted as long as it looked good and he stayed strong ...... a dog that's getting weak/tired will drop it's head below level , a fresh dog will keep it's head high , above level ............. i'd let him trot and walk , if he began to canter or gallup i'd stop him and return him to a walk , and see what he does as he continues ..... if he gets trotting again i'd let him go as long as it looks good , return him to a walk and see if goes back to a trot ...... a healthy strong sound 6 month old dog will have a lot more than a mile in them .......... a trot at the right speed is a very efficient easy gait for a sound properly built dog ...... it appears to me to be an easier gait than a walk , a good moving dog will prefer it over walking....... the most critical timeframe for hd is already past in a 6 month old dog ....... if your dog is just starting exercising at 6 months , you'll want to keep an eye on him for tired , tired is when they start banging them joints ......... best to start walking that 12 week old pup miles and let his strength keep up with his size , rather than wait until they weigh 100+lbs and then expect them to handle it ..... much easier with the first scenario ........ as for gaits , trot and walk is all i'd let them do , canter , gallup or pace are high impact ...... i don't know what breeds pace , but i've seen big wide dogs that for some reason wanted to pace , they roll side to side , it's a horrible , tiring gait , at least for any dog i've had ......
as l b skyline said , pulling is a great exercise , dragpull chains ...... again you need to recognize tired /weak and strong ........ tired and panting or slobbering have nothing to do with each other .......