I was thinking about this today as I went out for my first run since the 6 hour Dizzy Goat race I did on Saturday afternoon in Gretna, NE. Here in Iowa, a lot of us do not have an abundance of trails nearby like we read about for those who live in the trail running meccas out in the western states or even in New England.
But, good training can be done by mixing it up, especially in what I call multi-terrain runs. Today for example, I ran about 4 1/3 miles. The first 1+ mile was pavement. The next 1+ mile was gravel. Then I ran 1/2 - 1 mile on the local golf course. Then after a tiny bit more gravel and pavement, I ran maybe 1/10-1/4 mile on a trail, before ending the run on pavement. This constant change of surface breaks up the monotony of running and gets your body use to the various types of terrain that often come with trail running. You get use to stepping on and off various types of terrain. You find different paces depending upon surface and learn how to run to your advantages and/or work on your weaknesses.
The Dizzy Goat course for example has grass, trail and pavement...just like the loop I did today around our little farm town. Is there a multi-terrain run you can add to your weekly routine? Look for parks, golf courses, side roads, empty farm/jeep roads. Have fun exploring!