Trust me you have nothing to be worried about on the injury front and once you go high intensity you never go back

Here's a nice quick one for you to try which only takes 15 mins...

30 second intervals of each exercise, doing as many as possible, each round totals 3 mins, with 30 to 60 seconds break between rounds. Do 4 rounds in total.

Burpees
Jumping Jacks
Split Jumps
Burpees
Jumping Jacks
Pushups (sets 1 & 3) Squats (sets 2 & 4)

I usually only do a bit of skipping and stretching as a warm up for this, just make sure your quads are stretched. Then afterwards do an ab circuit and you have done a great, time efficient workout in under half an hour. If your feeling adventurous, do some bag work afterward. Now you have no excuses Sharla

A few of my mates have been lifeguards, pretty cruisy job most of the time, well paid; I'd go for it!