I could walk Boo, 6 yrs, for hours and hours and he would still be raring to go.
Gracie, 5 mo, keeps right up with him. Boo is smart and will let me know if it's too hot and they need to rest. He looks for a shady spot and will flop right down to cool off.
On work days we have to get up at 4 a.m., so morning walks are pretty much out of the question. Texas is pretty warm right now, 90+ afternoons, so we try to wait until 8'ish to walk at night. The sun is mostly down below the horizon and we do between 30 - 45 min. Combined with play time and time in the yard this seems to keep their energy levels down to a dull roar. I've also been know to take them to the local pet store and do 'laps' inside where it's cooler.
Perhaps if you could take Jaya out first for a very brisk 30 min walk and then have Jenna join ya'll for another 30 min walk. I think having Jaya with you will help to keep Jenna focused on moving forward. I personally don't think they need hours of walking, but walking combined with play is good. Gracie loves to play fetch and I can throw her a toy for nearly an hour before she's ready to move on to a different game.
The important thing is to listen to your own instincts and do what
you feel is right for your girls. Walking with your pack is important to their state of mind, but if Jenna is needing to lose weight, any type of activity will have benefits. Walking in a pet store if it's too hot outside, or one-on-one play time will be good too.
Good luck!!