Your boss will likely just be happy you're back! Shorter days for sure at first. Those shorter hours will give you the leeway to leave sooner if your knees get cranky. If your knees do well, you can always go for longer.
You are going back way too early, though. You already know we think 12 weeks is the time to give it a try. You are only a few weeks out (averaging your surgery and MUA). Impossible? Not quite (very little is truly impossible when it comes to the human spirit), but you might find your return to work very difficult. Is your work the kind you can do from home? Even early on, I was able to do a great deal with a laptop while propped in my recliner. Some of my husband's attorneys recuperated for months working from home. If work from home is an option, take it.
Some things to keep in mind:
* Elevating on a chair is nowhere near as good as elevating toes over nose. It's just better than nothing. Later in recovery, toes over nose is less important.
* Your brain is also recovering from this surgery. Sounds weird, I know. But between the bodily stress of healing, the medications, and your emotional upheaval, your brain may not work as well as you want it to. Some people are more affected this way than others, but do keep this in mind.
* It's not just the sitting at the desk, also factor in your commute (type and time), walking to and from your desk, and whether the stress at your job will add so much to your energy drain as to wipe you out. Exhaustion helps neither your recovery nor your work performance, so if it becomes an issue... cut back.
I know what you mean about needing to work to keep your boat afloat. While a working single mom, I worked with full-blown pneumonia because I couldn't afford to miss a paycheck. We do what we have to do. If you can, though, take a few more weeks for your recovery.