I'd sharpen the blad on that mower, set it at max height, and take it slow. After the first pass, cut only half width "rows" and take it slow. Once you get one cut, rake, then mow again on a lower setting. By this point, you can take larger "bites" and move faster.
I had to do this the other day with my electric mower. I have a much smaller yard though.
Tilling it under and starting over might work, but it may also open your lawn up for a weed invasion. As long as you keep a nice thick carpet of grass, it's hard for weeds to get a foothold, but by tilling it, you'll open it up for every weed seed dropped by the breeze, birds, or animals. I had no weeds at all in my front yard until a drought and my laziness let a large thin patch appear (lawn pattern baldness?). From there on, it's been a battle to keep weeds out. I'm almost back to my previous state, but it's taken a lot of chemicals and manual weeding.
Chris
