is definitely playing far too much as a very beginner, especially if that 2 hours is in an uninterrupted block. Excess enthusiasm can get us oldies into lots of trouble... ;-)
You can probably do much the same total hours, even at your level of physical experience, if you split it up a bit: but first you should let your body heal as much as it can. The 2 hours block should be cut into at least 2 sessions at first: you can increase the session lengths when your body has adapted. You can cut the time into 1/2 hour sessions, or even 10-15 min sessions, with more sessions, especially till your body catches up with the increased/different stresses. Listen to your body, if stiff or sore, back off. Us oldies do not have the resilience of youth that allows the body to cope with such sudden changes in physical stressors, so take it easy until you catch up.
I never could figure out WHY only a single strap - maybe on the very smallest/lightest boxes... especially if played for any length of time at a stretch.
Perhaps Bob, or some of more knowledgeable writers could explain WHY (other than just tradition) the instrument had only one strap.
A Piano Accordion would be damn near impossible to play at more than a very minimal level with only a single strap. I know experienced players can just grab the box, fling it on a knee without using the straps, and get a quick tune out of it, but that's not they way we try to play it when we are planning to be 'serious'... :-) And with larger boxes, you usually need the 'back strap', but I can't reach around to hook up or release the normal commercially supplied ones.
If joints are moved out of their central 'neutral' position and held there for long periods of time, especially with any force, you are asking for pain and damage, joint & muscle wise - this is one of the concepts of Mr Alexander. Musical instruments held 'naturally', means that the joints are in as 'neutral' a position as possible. Just grabbing the things any old how doesn't necessarily achieve this: because of normal slight differences in anatomy, everybody's 'neutral' positions may differ slightly.