I used to be fairly age-bracket competitive in tri's back in the early 90s. There is no substitute for training on any one aspect of the overall event. Your CV fitness will be fine from the cycling, but the muscular endurance is different enough between cycling and running that you can't slack on one and expect to shine on race-day just because you did twice as much of the other.
If you've hurt your knees to the point of needing surgery, don't run any more. If you crave the competition, pick a cycling discipline that suits you, and go after that. I busted the snot out of my legs putting in 80+ mile weeks of running when I was younger. I'll ride 250+ miles in a day now, but you won't catch me running for anything.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.