You can rebind
set-mark-command to anything (C-. in the example)
(global-set-key (kbd "C-.") 'set-mark-command)
(define-key 'global-map (kbd "C-.") 'set-mark-command)
Alernatively, without having to rebind anything:
it's also bound to C-@.
You can exchange point and mark C-xC-x -- if you're not using the current mark already it'll be the same.
You can mark the paragrah, or org-element with M-h, move the point to first line you want, then reduce region exchanging point and mark first, and moving the point again to the desired end. Sounds far more complicated than it is, but sometimes it's faster than any other method.
Sure there are more ways, but I do use those.
Straight to the example, to select directly only the src block contents you can use
org-babel-mark-block which is bound to C-c C-v C-M-h. Sure not really the most friendly shortcut for the memory muscle.
Thanks to bertfred for pointing it in this answer