re: Nagging tendon pain

Sounds like you had a biceps strain. Strains, much like muscle soreness after training, are tears in your muscle/tendon/musculotendinous junction, except with strains they are too large for your body to heal by making new muscle/tendon so it fills in the torn area with scar tissue. It sounds like to me that you are having some issues with the scar tissue that was laid down. Unfortunately scar tissue is laid down haphazardly and often times you get that nagging pain with it.

You need to decrease the tension of your bicep. Stretching is good and I would suggest myofascial release as well(i.e. massage gun), followed by medium intensity eccentrics or heavy slow resistance (HSR), and finish it off with Transverse Friction Massage (TFM) at the point where you feel your pain. TFM and myofascial release can be done multiple times per day, but I would recommend you do the eccentrics and/or HSR every other day.

ETA: What kind of mountain bike do you have? Mountain biking is the only thing that has made my heart skip a beat since my time overseas. I've never had something humble me (I consider myself pretty fearless) more than mountain biking.