I was never advised to exercise or do anything other than my pre op liquid diet, but the day i got on the consultants scales i felt so ashamed i turned over a new leaf there an then cutting back on the amounts i was eating, opting for healthier choices instead of the naughties and increasing my exercise levels, whether it made any difference who knows, but i felt better and more comfortable going into my op, had a positive mental attitude about it all and because i'd really cut down i didn't find or struggle with the pre op liquid diet.
I'd say that any exercise or dieting you can do pre op has to increase your chances of a faster recovery and decrese the risk of things happening whilst under the knife ! and any increase in your metabolic rate will aid post op healing.