I remember crutches well :(
Tip 1: pad the crutch handles well. I also bought cyclists gloves which are padded, which helped.
Tip 2: for goodness sake ask for and accept help. Ask if somebody from work can pick you up/drop you off at the station. How are you managing at the other end?
Get a close friend/family member to help you shower. Dignity is for those who don't want to get washed 😁
Who can you borrow or buy a cheap plastic chair or stool from? That's what you will need for a shower, or if you have a bath, a bath seat (can be as simple as one that looks like a plank across both sides). With either, sit down then swing legs in/out - MUCH safer.
Bin bags and tape work as well as posh cast covers.
It getting looser will happen because your leg was swollen with your injury when it was fitted. Your leg will likely swell up when you do too much or haven't had it elevated enough (consider elevation at work, you will want it on a proper chair rather than little tiny stool). You need the extra room in your cast to allow for that swelling for a little while, if it swells without room that's painful and not good.
Do readjust your expectations. Your shoulders and arms will hurt, yes. You will get out of breath, and you will get TIRED because that's what happens when your body is healing. Don't try to go back to work too soon.
Dig out a rucksack or cross body bag and a flask.
Did I mention ask for and accept help?