In U.S. units, good useful gearing for touring is considered to top at 100 gear-inches (see http://www.sheldonbrown.com/gears for a gear calculator). As it comes, the Trek 520 has gearing up to 127 gear-inches. And the low gear, being at 25 gear-inches, is good either for a young person with good knees, or someone who never climb hills steeper than 8-10%, or someone who doesn't load too much the bike... but not when two or more of these factors are combined!

If you mostly commute or tour with light loads, you could "improve" the gearing with something like Sheldon's Cyclotouriste 13 or 14 -- http://www.sheldonbrown.com/k7.html --. It gives you closer ratios near the top (good on the flats) by getting rid of those "useless" high gears. However, you can't go much lower than you have, except by changing chainrings. BTW, you can mount your own cassette with cogs taken from 2 or more cassettes.

For your information, I have 44-34-22 chainrings and a 12-14-15-16-17-19-21-25-32 cassette on mine. It gives me a high gear of 99 gear-inches... whcih I rarely use. Even my next highest gear, at 85 gear-inches, is rarely used. And while it might seem low in regard to what's commonly available these days, if I spin pedals at 80 rpm, it allows me to pedal at 33 km/h (top gear at 80 rpm would go to 38 km/h).

Regards,