Hello, speed work is a higher risk of injury and so is fitting in more runs a week as you need time to recover. Cycling is great, get yourself a road bike or hybrid and go and an hour twice a week. That gives you two none training days.
Sunday run and so on and so on.
Gradually work up your distance from 5km at 1/2km a week maximum and gradually you will get stronger.
Once you get to 10km you will be stronger and then possibly ready to ask your speed question again - say in three or months time.