No, Garyyou won't gain a thing, life-of-shoe-wise, with an aftermarket insole such as the Scholl's gel insole. Or, more properly, the Magna Energy Gel Insole ($17). You'll be cushioning your own bones and joints more, but the midsole and outer of the running shoe still will take a pounding. What can
extend the life of a running shoe is something you already use through your orthotica foot stabilizer of some sort. Something like that will help control over- or under-pronation, helping reduce excess abrasion on one part of the outer or another. For those who don't need a prescribed orthotic, Superfeet ($26available in several styles) work well.
The unspoken question here is: How long do running shoes last? The generic answer: Six months, depending of course on how many miles you run each week, your weight, the terrain, and so on. Most running shoes still use EVA as the primary cushioning layer in the midsole, and the stuff just isn't that durable. It will look fine for years, but its cushioning greatly dissipates in that six-month span for most people. Polyurethane is more durable, but less cushy. Air inserts, gel inserts, and other proprietary gadgets work well, but still are typically encapsulated in EVA or something similar.