Hike: n. a long walk especially for pleasure or exercise; v. To go on an extended walk for pleasure or exercise.

Okay, now that the lede's out of the way... Webster 1913 defines hiking as "to go with exertion or effort; to tramp; to march laboriously." You might think that this is simply a change in tone as the word evolved through time, and perhaps it is, but there's something else going on here. The word hyke first appears in written English in 1809 in a letter from English organist and composer Samuel Wesley to a friend:

"Adieu for the present,-- we must contrive one more Pull at Surry before I hyke over to Staffordshire. Kindest regards."
The Letters of Samuel Wesley: Professional and Social Correspondence, 1797-1837, By Samuel Wesley (Letter sent to musician Benjamin Jacob, Sept. 4, 1809)

This letter was written in Camden Town; Camden Town is 150 miles from Staffordshire, and Surrey is 160 miles away (in case you are wondering, it is only 30 miles between Surrey and Camden Town). Hyke would probably have been in fairly popular usage at this time, in order for it to be used in this letter without clarification, so it is entirely possible that Wesley is using the term figuratively. But that's far from certain, as hike was probably already collecting some interesting connotations.

The The West Somerset Word-book (1886) claims that 'hike' simply means 'to go', but notes two idiomatic usages: 'hike off', meaning "To skulk off. To slip away, like a rat leaving a sinking ship"; and 'hike out', meaning "Turn out; get out; set off."

Meanwhile, the 1894 U.S. Yearbook of Agriculture used hike in the more modern sense: "Short and frequent hikes, requiring no preparation or special equipment, should be encouraged for most people. The longer hikes, usually requiring advance planning and preparation, may be somewhat more challenging."

Webster's examples might be influenced by the American usage of hike to mean 'pull up' and to 'rise up', first used in written language in 1873. (he gives this usage a mention as well, with "To move with a swing, toss, throw, jerk, or the like. {Dial. or Colloq.}") Or they might be the most exacting and appropriate examples of modern usage in 1913 (although the US department of Agriculture might disagree).

Even today, hike has a range of meanings, from a difficult or laborious trek to a stroll through the woods, and the exact meaning relies heavily on context. This was evidently even more the case in the late 1800s. Despite the impression that hike had a clear, established meaning in 1913, Webster is probably obscuring the fact that it was an informal, slangish term that still wouldn't have been included in many dictionaries -- for example, it did not appear in the 1933 edition of the OED.