Hi Jane.
Running around the Garden on her own is no problem, she can regulate herself and stop for a sleep anytime she likes. At that age mine get around 10 minutes walk on the lead only, Twice a day if you like providing there is plenty of rest time in between. As for the run around the field, don’t forget that as far as work is concerned, following you is to all intents and purposes the same as walking on the lead in that she can't stop for a sleep until she gets home. If you want to give her a run in the field I would make it a 10 minute period replacing one of the lead walks. We all want to go for a nice long walk with our dogs but, please be patient, give the bones time to harden and you will have a companion for your walks for as far as you want to go. Gradually increase the distance over the first year and you will be giving her the best possible start in life.
Best wishes, John