Listen to Erica Nelson recount a few of her thrill-seeking adventures, and at a certain point you can only shake your head. How else can you respond to somebody who tells you about her two-month bike tour around the Baltic Sea? Or her 14,000-mile motorcycle trip along the Pan-American Highway? Or the weeklong dogsledding excursion that she casually tacked onto the end of a backcountry ski trip in Northern Sweden?