Sounds painful. I am living in the tropics now and, if it's not a water based activity, I get out early and finish early to avoid the brutal heat of the day. It's just too hard to carry enough water to stay properly hydrated. Obviously, that's not an option for you.
You'll get all kinds of advice, but one thing that makes a big difference to performance over time and particularly maintaining muscle mass is getting adequate protein. Athletes (and you qualify as an athlete with this job description) should be getting a gram of protein per pound of body weight per day. That's actually pretty tough on a vegetarian diet. Note that the recommendation is the actual protein not the weight of the food, so one egg has 7 grams of protein. If you could switch your metabolism over from the standard carbo crashing junkie one to burning more fat for fuel you would be able to eat less frequently and not have blood sugar swings, but it usually takes people a couple of weeks - at least - to adapt to burning either body fat, or fat you eat instead of relying on glucose.