Strength

A large part of our speaker's attention (and hence a large part of this poem) is devoted to describing and admiring the tough work that pioneers do. Their feats of strength—chopping down forests, digging mines, etc.—and their courage in the face of rough terrain and danger seem to represent our speaker's ideal lifestyle. This is no country for old men. Or sissies.

• Line 19: The first half of this line gives us a handy little metaphor that equates the actions of pioneers with taking hold of the world (and equates the world with something that can be seized). The second half of the line describes that world and tells us that it's a world of work. This metaphor is one of many instances in the poem that suggests to us that toughness, strength, and vigor are to be admired.
• Lines 21-23: This implied metaphor, of pioneers as a sort of army, is a central thread of the poem. On the one hand it's a way of showing a sense of unity. But armies also have connotations of bravery and strength. The imagery here of them throwing themselves up steep mountains and conquering the unknown could be filmed as a montage and labeled "Pioneer feats of strength." 
• Lines 26-27: These lines are chock full of wordplay. The anaphora (starting consecutive phrases with "we the") presents a kind of lyrical strength through its repetition, even while the scenes described in the phrases are of acts of physical strength. The words "piercing" and "virgin" also imply a sexual metaphor, with the men penetrating the earth. And, finally, that sexual metaphor implies a personification of the earth, since, strictly speaking, the soil can't be a virgin (but through personification it can!).
• Lines 93-94: These rhetorical questions help our speaker engage the reader while he presents his image of his non-ideal. Eating and sleeping a lot, behind locked doors, is just about the opposite of a feat of strength. To our speaker, it's a display of complacency and feebleness.