I’d like to address the issue of wearing cross-trainers for your marathon preparation. The two issues worth addressing are the cost/economy of having a multi-purpose shoe, and the technical aspects of a cross-training shoe versus a running shoe.
On the technical aspects of the shoes – Cross training shoes are designed to perform as well as possible for a number of sports, versus running shoes are optimized for running. The primary difference is that cross training shoes are designed to allow participation in court sports. This design “feature” means that the shoe must be laterally more stable than a running shoe, and have an upper that is sturdy enough to take the side-to-side forces. The result is that the upper of a cross-trainer is heavier, and does not breath as well. This will cause your feet to be more damp from sweat, which can lead to fungus issues. Blisters can also be a problem, as your feet will be more damp and the cross-trainer shoe upper is heavier and stiffer than a running shoe. Further, the midsole of the shoe will be thinner AND firmer to permit a lower center of gravity and increase stability to reduce the occurence of lateral ankle sprain. The thinner and firmer midsole will mean that you will not get as much running milage out of a pair of cross-trainers as you would out of a pair of running shoes before the shoe feels hard and lacking adequate cushioning.
The most important technical aspect related to the shoe choice has to do with your biomechanics. Relative to the types of running shoes available (cushioned, stability, and motion control), the cross-training shoe most resembles a motion control running shoe. If you are an over pronator, this resemblence is good. If you are an over supinator, you will be very injury prone running in a cross-trainer. This is because a person who over supinates needs more cushioning than is offered in cross-trainers and motion control shoes. So the obvious question is how do you know if you are a over pronator, neutral, or an over supinator. I have some very detailed information on this topic on my website. See the link below, and then click the link titled “Information for Runners Regarding Shoes…” in the navigation bar on the left. The alternative is to go to a good running shoe store that offers video gait analysis and have them video/watch you run. They can then make a shoe recommendation appropriate for your biomechanics.
On the cost/economy issue – I can only assume the reason that you want to wear a cross-training shoe is so that you can reduce your total shoe purchases. My assumption being that you do other activities (gym, basketball, racket sports?) and you would like to buy one pair of shoes for all activities as opposed to multiple sport specific shoes. If these assumptions are correct, you may not actually save anything. For sports where wearing a shoe that offers shock protection is important, a dominant factor in the wearable life of the shoe is midsole breakdown. That is, the midsole of the shoe (the soft part of the sole that offers the shock protection) breaks down and no longer offers shock protection usually quicker than the upper or outsoles will wear out. This is the reason that many runners will use the rule of replacing their shoes every 300-500 miles; where “miles” are really any activity that you are wearing the shoes. My reason for pointing this out is to demonstrate that shoe “life” is mainly a factor of time in action in the shoe. So you can buy one pair of shoes and replace them very often, or buy sport specific shoes and replace each pair less often. So I do not think there is a valid economy issue.
In short – I see no advantage to running in a cross-trainer and you will possibly be at increased risk of injury.