What is the ground speed of an unladen Ent?

[Kalimac]

This is one which my husband thought up (in the airport, while we were trying to stay awake and of course talking about LOTR [img]smilies/smile.gif[/img]) for people who are trivia-minded and good at ME geography, since we're both fairly poor at it. We were reading the bit in TTT where Treebeard tells Merry and Pippin that he has taken them "seventy thousand Ent-strides" during the course of one day. We don't know exactly how long it took them to get to Wellinghall, but Treebeard found them in the morning and they arrived at evening, so say twelve hours. If you do the math on that, Treebeard is taking about 1.6 strides per second which is about normal for a human walking briskly. Assuming that his stride is about five feet or so, he's covering a good 70 miles in one day. This was a little odd to contemplate mostly because I'd always pictured Ents as walking rather slowly, because of their size and also because of the painstaking description Tolkien gives of Treebeard coming slowly down the "staircase" - it seems like they wouldn't want to do anything too hastily, including walking. Also, how big is Fangorn exactly? If it's seventy miles between the edge and Wellinghall, how far is it to Isengard?

Sorry if the question is a little confused, basically I'm just curious about whether first of all the Ents are supposed to go that fast, and secondly if the distances they would cover at that speed jive at all with the geography on on the maps; it's a bit hard to tell.