Loss of power in a 1999 Chevy Blazer 4.3?

I just bought a 1999 Blazer 4.3 and for some reason it has a hard time picking up speed from a dead stop! The transmission is not slipping and the cat converter is not clogged up! It does have high miles, 177,000 to be exact! But I had ford trucks with more miles that more power! Is the engine just tired or it it just geared slow? The tires on it are 235-75-15 but I can't see that would make any difference, would it? The engine sitting and running sounds like a brand new one and it has all kinds of power once you get started out going down the road! The only thing I can come up with is just too many hard miles on it! What do you guys think?

I would start simple, check the ECM for any codes that might be stored in there. Then start with a tune up. The 4.3 engine are know for failing distributor caps. Also could be a clogged fuel filter.

Get a base tune up. The Blazer probably has not seen a mechanic in a long time.

The other answers are more likely, but I once experienced the same "lack of power, difficulty accelerating" right after replacing a Master Brake Cylinder. Seems the new one was faulty. Front brakes weren't releasing completely.
Just a thought as you say you recently purchased it and may not know the full history of repairs.
Take it for a drive, apply the brakes several times, drive several hundred yards, get out and (carefully) feel the rims. If any are really hot that might be your problem. A long shot I admit.