Start with some pick ups in your normal runs. These are also called strides in the running training world. Accelerate up to a relaxed fast effort for 10 seconds and then slow back to your normal easy distance effort for 2-3 minutes. Do this for 5-6 times in a run and assess how your leg feels the next day. Gradually introducing speed work into your plan this way will allow you bones and connective tissues a chance to become stronger under the increased load without prolonged stress. We have used this approach for years with good results. If after 4-5 of these pick up runs you’re feeling stronger take them out to 12 then 15 seconds. Then increase the number of reps up to 8-10 in a run. If you can handle that for 2-3 months you’ll have the confidence to do some extensive intensity in Z3 moving to Z 4. It’s all about allowing the body to adapt at its pace not a pace you impose on it.