As handyman I find this a rather common problem ..
If one glider is brittle, the others are as well. The track may be dry and pitted. Customers often say ' oh I have a box with a mixture of old gliders, would you like to have a look'.. Personally I wont get involved in fitting old ones, because when they break soon afterwards, the customer will say .. 'he didn't do a very good job'.
There are many sizes of the glider you show in the link and they are all suited to different size tracks. If you fit the wrong ones, the curtains may pull the gliders off the track, or they might not even slide at all.
My advice would be to replace the track and the correct size gliders will be included in the pack.