A 1st life character needs 8,500,00 XP to get from level 1 to 28. According to the above linked Wiki page wilderness areas can give over 8,019,379 XP without repeating rares, so with XP boosts and repeating rares it should be theoretical possible to get to level 28 with wilderness areas only.
The only problem (except from time and finding the will to live while doing it) may be the level distribution of the wilderness areas, but even if you are over-level for an area you can still get some XP.