This is fair, and I address it in next chapter, but maybe should have included here as well. Humphrey's appearance takes a toll after the surgery, but he has started to rebound by March -- putting back on weight, etc. He is not undergoing chemotherapy, so while the cancer is putting him in pain, he is not experiencing some of the more pronounced side effects of cancer treatment, notably hair loss. This sort of mirrors Humphrey's appearance IOTL. Carter notes in his WH Diary that HHH looks "near death" in December of '76, but then, a few months into '78, he remarks that Humphrey looks far better than he did back in December, and even that he expected Humphrey to have more time in him than he'd thought.
As to your point about his schedule, I think the Seventies were an interesting balance between Television Age and Social Media Age. Humphrey is rallying for some public appearances -- press briefings, cabinet meetings, etc. He's taking some staff meetings in private study in the Residence -- and he's reaching out to Congressmen and Senators via phone. Plenty of presidents have kept a light work schedule.
I would also argue that with something like this a patient is bound to have "good weeks and bad weeks" -- there are enough times when Humphrey can go a few days of an active schedule to allay concerns. Then, he may spend a day or two in bed but working via phone only to resurface for a public appearance and a heavy work day in the Oval. This kind of a roller coaster of his condition helps him keep questions and concerns at bay.