I'd be very curious as to how the Guinness "researcher" arrives at his figures, especially since I always though Hugh Downs' claim was an empty boast. Downs probably counted the five years he spent as Jack Paar's sidekick, where his actual contribution was minimal.
And frankly, Regis' resume isn't filled with the sort of long-running series that would make him the all-time champ. Sure, he's been around a long time, but so have a lot of other veterans. Unless he was doing several shows at the same time for long stretches of his career, how would his total be any different than, say, Bob Barker's?
Meanwhile, Katie Couric is still a kid by comparison, and is probably approaching those totals by virtue of up to fifteen hours of on-air time per week. There just seem to be too many variables for anyone to make a meaningful claim to having the all-time greatest number of hours on TV.