I am currently experiencing with tempo runs for my duathlons. After 5 min of slow running I was doing 10 min. of 85-90% of MHR with 5 min recovery, twice in a row. I am now comfortable and tried 15min. tempo runs. Should I stick to 2*15 or 3*10 or something else considering I am compeeting in 5-30-5 races ?