+1 for what Kev said. Afaik, as long as it is separated, you will be fine. My mains cable, is inside armoured cable, which is dug 12-18" under the ground.
Irrespective of it being buried, or loose, ie shown, between the 2 buildings at the top, I would personally thread the cable, through a hosepipe. Especially in the garden. You never know, when someone may put a fork, or spade through it, or the cable degrades due to the elements. Digging up the garden again to recover and relay the cable would be a pita.
If it was inside a hosepipe, it would be a simple matter, or tying a piece of string to the new cable, attached to the existing cable, and then pull them both through.
Saying that, if it was loose in the air, again I would personally thread it through a hosepipe again. Especially with the elements, frost, snow etc. My cable is approx 30m from the front of the house too, where the router is, and is strung from the house to the workshop. It is a little unsightly, as I have 2 cables run down here, one for the telephone, and the other for the network. I used Cat 5e, for both. When my phone cable needed replacing, it was a bit of a pain, to replace, as they are both cable tied to each other. So ended up replacing both at the same, time.