"Endless" as an adverb without -ly?
Solution 1:
Yes, endless can be an adverb, as a good dictionary will tell you. The OED says it merrily serves in either role, adjective and adverb, and cites examples dating back to King Ælfred in 888 ᴀᴅ.
Here, though, is a more recent citation for your regard and appreciation:
Yea, one lies dead for millions dead !
On red spot in the snow
For one long damning line of red ;
While exiles endless go —
The babe at breast, the mother’s head
Bowed down, and dying so !
By Sophie Perowskaja in The Californian, Volume 4.