It may be difficult, but the good news is that Dungeoneering is completely free (unless you are leeching like Stui suggested.. although leeching is much, much faster).
F2P dung sucks though because you are basically handicapped. The maximum complexity you can do your floors on is c4 I believe, which severely affects your exp at the end of the dungeon. It sucksYou can definitely do 70 dung though, it will just take awhile.
It would take over a week playing 6+ hours a day.
How much quicker would members dungeoneering be?

