the only mistake you have made really is that your machine is now limited to that size screws/rails, where as the norm is to design how much working space you need and then work out the screw and rail lengths from that.

also you have bought from what looks like the seller linearmotionbearings (either that or they all use the same picture) and had you been on the forum a little longer you would have eventually found out that he is a regular supplier to a lot of folk here and is more than happy to quote you prices outside of ebay and send you a paypal invoice which reduces his fees and the price to you (best special price to you) so you could have got them cheaper.

you'll get a lot of help here and they are a good bunch of guys... welcome :D