That would be too much for me in one go personally. Jacks are prone to luxating patella.
It also depends what your doing, free-running on grass or sand is not going to be as punishing as the same distance of lead walking at brisk human pace on pavement.
I don't really 'walk' dogs for exercise until they are about 6-8.
Before that is for socialising, getting out and about, teaching lead manners etc. This is the time for brain training (which ideally should tire them out more!), bonding etc.
5 minutes per month of age is a rule of thumb I've heard - so 40 mins is too much really. As CC says, the main thing is socialisation at this age - go to different places so the pup sees as much as possible. You can use energy up with basic, fun training - mental stimulation is just as good as physical exercise IMO especially for babies
Yes my vets told me 5 mins per month of their age - so yes I think that is a bit too much. Prob wouldnt hurt as a one off but certainly not as a routine/regular thing until he is a bit older.
Really the 5 minute rule depends on the dog and the breed, however 40mins is a bit long for a pup as a rule of thumb. One thing to bear in mind that mental stimulation will knacker out a dog much quicker than physical exercise, so if in the evening he is getting a bit energetic, a bit of training of his sits, lie downs, stays etc is always a good option provided he get a treat.
Otherwise you can always teach him the 'find it' game. Get him to recognise by name a toy which he loves, a ball, a teddy etc and start off hiding it in really obviously places, rewarding him with a treat if he finds it. As he progresses, make it more difficult (but always within his reach and limits of the house). Currently I hide about half a dozen balls all over the house and it keeps my dig occupied for at least 20mins after which she then has a nap! And peace for me ;-)
Walks off lead are actually less physically demanding for the pup as they're self regulating their own pace. A walk out on lead for the same amount of time would be much more demanding. Bonnie (golden retriever) got twenty minute walks as a puppy - took about 5 mins to walk to the woods, got 10 mins walk off lead, then 5 mins to walk back home again on lead. We always let her off lead from day 1 as it helped with training recall.
I stick to the 5 mins per month rule, and never take them for a walk before they are at least 6-8 months, just a little sniff in the woods and a bit of socialising.
Depends if it is hooning around a field on grass with other dogs (that might be too much for a pup of that age, depending on the breed) or going at a steady pace for one hour.