I am an ex smoker (male, 26, 173cm (5'8")) – smoked about ten years with varying intensity – and since some month really getting into sports. I started mountain biking, shortly after I began swimming and just got into road biking. Biking is a lot of fun, but takes much time and preparation, I usually bike for two to three hours, and need at least one hour of preparation before (planning the route, prepare drinks, get dressed).
So when I walked the dogs last Thursday, I spontaneously decided to run the route instead of walking. It was a 3.8 km route (2.36 miles), mostly cross-country, up and down a small hill. Because one of the dogs is mainly concerned about digging up potatoes and checking out dung piles I often had to run on one spot to wait, but I tried to keep moving.
Up until yesterday (Sunday) I had heavy legs and walking stairs was a bit painful. I was quite surprised that I can cycle 100 km without major pain in the legs, but those few kilometers of running killed my legs.
Anyway I decided to run again today and decided to go for 5.1km today (3.16 miles). I looked up what time I had to reach according to the Deutsches Sportabzeichen List (German Sportsmedal) and it is required to run 5km in 23 Minutes for my age.
I actually got the first half of the course (cross country again, but mostly flat) in time, but after that I had to pause and walk. To avoid stitches I had to slow down with my pace and it took me (with walking breaks) 33 Minutes to finish.
My short term goal is to reach this 23 Minutes on this specific 5km course (without pausing of course). My long term goal is to be able to run longer distances, not necessarily speed. Still my main goal is overall fitness and mainly biking.
How often should I be training per week to reach my goals regarding running and would it be a problem if I eg run at one day, bike the other, and run on the next again?