It is very difficult to engage in exercise when other health concerns are at play. If exercise is very difficult with your present state, focus on eating right. Most weight loss specialists agree that weight loss has more to do with diet (up to 95% diet) versus exercise. Take a look at this article for advice: http://www.wikihow.com/Develop-Healthy-Eating-Habits
Commit to a 7-day meal plan. Create a 7-day meal plan that covers 3 main meals (breakfast, lunch, dinner), scheduled at the same time of day, and 2 small snacks (between breakfast and lunch, and lunch and dinner), scheduled at the same time of day. A set meal plan will ensure you eat at a consistent time every day and do not skip or miss a meal. Consuming about 1,400 calories a day, combined with exercise, can help you to achieve healthy weight loss.
Back to the tricep pushups and really activating those tricep muscles for toned arms. If you need a rest, that's totally cool. Just take a break in the up position and maintain that plank form. This way you are still working your arm muscles just by holding your body weight up. So that's totally fine for some of our beginners out there. For those who are more advanced, we want you going as hard as you can and trying to hit that 15 to 20 reps. You're doing great.