I'm 52, I drop to one knee (left) to dig anything deeper than 4" ...for shallow targets, I do a wide leg stoop, think of a girafffe taking a drink, and rest both of my elbows on the tops of my knees and stab at the targets with a screwdriver...Maybe try to break up your day, and kick chips in a totlot for a while, or work a sandy area with a long handled scoop.?
No problems with knee pain, but glute muscles get a work out. I think the quicker you can get a target and get yourself back up vertical the better off you are. Also, depending on what you are after, Maybe going after deep silver and digging deep holes is something you should only do sparingly? Hitting shallow targets fast is fun and easy, it usually only takes 10/15 minutes to work a totlot, then you are on to the next one, or sweeping the sidelines of a sports field for shallow freshies,...I carry NO gear except a screwdriver, so maybe even the weight of a toolbelt is adding to your pain?
Life has been hard on us, and I'm sure we are all busted up to some extent at this point, I think this hobby has added 20 years to my life, hope this helps.
Mud