Unfortunately it's too hot in the summer to walk dogs during the day, but a few short walks a day can be equally beneficial (if not more so as dogs have little - if any - sense of time) as one long walk. Take your dog out for 10-15 minutes early in the morning, again around 7pm when the sun goes down, and again around 10pm when it is a bit cooler. It is still hot and humid even at those times, so keep a close eye on your dog and if he/she looks to be struggling, take him/her straight home.
Try to think in terms of how many 'highlights' a day your dog has. Food is usually one of these, as is contact with its owner, games and/or toys, chewing, digging, a training session or exercise (amongst others). So having two-three walks a day instead of one gives the dog an extra highlight or two a day. This can be invaluable in boosting a dog's mood state, thereby making him/her less restless or frustrated at being cooped up inside all day. Chew toys are also excellent. They keep dogs busy, help them relax and help keep their teeth clean.