Walk lengths or time

hi peeps just after general advice really. just starting to take my massweiler (bullmastif cross rotweiler) on longer walks than the 10 minute walk to school. what is the general rule to how long and how far ect. he his just coming up to 19 weeks and is 16 and half kg/s. there is a very local wood near me with very little or most of the time no one in there walking there dog's but the full walk i can estimate at about an hour to an hour and a half, i dont want to take him on the full walk if its to much for him, been working on his behavour for the past 8 weeks and im very impressed with his progress, stopping at road side without command stopping jumping up at people and knowing other dogs are not a threat to him. i can let him off lead in the woods and he follows me everywere and returns on recall, but at the end of the walk to get the lead on his harness he thinks its a game of chase but i'l get on top of that lol.

A good rule of thumb is one minute walking per week of age. Especially with large breeds you could be storing up problems with joints in later life, if a young puppy is forced to walk longer than this.
Running around the back garden can be counted separately, as the pup can rest when it gets tired. Out on a formal walk the puppy will keep going to keep up with you, even when it gets tired.
Even if you do not think 19 minutes is long enough to tire him out, extra time playing training games which are not as active will do the trick, as mental stimulation is as tiring as physical stimulation.