I used to be able to ride my bike for miles and do spin classes with no problems. I stopped when I started working a graveyard shift. It entailed sitting for hours on end. I did this for a few years. I am no longer able to ride due to severe hip/groin pain, primarily on the up stroke. Do you know of any specific beginner exercises/stretches that I can ease into to start alleviating this problem?
It sounds like the pain in your hip is coming from your hip-flexors.
This group is comprised of the: Illiopsoas; Sartorius; and Rectus Femorus. Their purpose is to: Flex your hip joint; Flex your trunk forward; Pull your thigh upward; and to help stabilize your lower body (at the pelvis).
The following 2 links have more in depth descriptions with videos to describe the basic stretch (as it can be hard to conceptualize via images). Some info is repeated