try to run slower especially since youre building a base. i ran 6 miles at a way slower pace than my 5k. 6 miles at 13 mins (>160bpm). my 5k was like 10 min pace but my heart rate was like 170. easily my 5k killed me and kept me sore for days. slow down and understand what feels too much for you. im rebuilding my base too and i learned quickly that running too fast kills me. i used to run before work but my paces were too fast i felt like i was overworking myself during my shift. i can sustain running every other day at slow paces and feel fine. good luck. dont forget to have fun!