Emily if you live in a flat area do you have any office buildings with large staircases?
Try 20 minute warm up and then tw staircases with single steps then one with 2 steps and walk one to recover then repeat until you get to top of building or you have done 20 minutes....or you die.If you reach the top and you havent done the 20 minutes go down in the lift and start again.
It works my mate works in Canary Wharf and does it once a week.
You also need to strengthen your hip flexors and hamstrings, so body weight lunges deep squats and burpees. Core and stability muscles, planks, supermans (for lower back) knee elbow crunches.
1 stairs session a week and 2 strengthening a week
That will cover it