My rucksack, two laptops, 2 power bricks, sandwiches and the other niff, naff and trivia required for a day in the office weigh one eighth of what I do; or, if you prefer, about the same as a v. light top end road bike.
That is my current excuse for my lack of speed. It has absolutely _nothing_ to do with advancing years and a hefty reduction in mileage of late. Oh dear me no.