The terrain earns its name. It's a random, massive scattering of little rocks - tricky to navigate, especially on your hands - that's pretty much devoid of vegetation. And, above all, it is steep! We are now just below 16,000 ft., having climbed 2,000 ft. in merely four hours.

Tomorrow we cross what's known as the alpine desert, but the mountain didn't wait 'til then to dole out some punishment: altitude sickness hit the team really hard today for the first time. I was lucky to escape the worst of it, but David and Alex got slammed - really bad headaches, nausea and fatigue. It was a real struggle but they're doing OK now, after some medication, eating and getting some much-deserved rest.

While the altitude sickness may have been inevitable, we've been pretty well prepared for most other challenges. (Months of preparation and research will do that!) Tamara, my good friend and colleague at Free The Children, climbed Kilimanjaro a few years ago and was one awesome resource. She shared loads of helpful info when we travelled together to Manchester, England, recently for my BBC Breakfast and BBC Manchester Radio interviews. She told me to drink lots of water (a minimum of two litres a day, no matter what), to sleep with a hot water bottle and to bring wrap-around sunglasses to protect my eyes from sun damage (especially for summiting).

Most importantly, the day before I left, Tamara pulled me aside and told me how proud she is of me.

Tamara is but one of the many amazing women in my life who I've been inspired by and fortunate enough to learn from - like my sister Annie, who taught me, among many things, the importance of taking the occasional risk. Safe to say I've taken that to heart, considering my current situation - climbing above the clouds with the world watching.

And I wouldn't even be up here without the woman who gave me life and taught me to be, above all, a compassionate human being: my Mom.

As David said yesterday in his Father's Day message, the higher we climb, and the farther we get on this adventure, the more we miss the ones we love. So true, but no real shocker. What did impact us today was the altitude sickness, the bitter cold and the steeper terrain. And even though we also discovered a renewed sense of determination, I think we are all awed by what we still have to accomplish.

Tomorrow we trek up the alpine desert. The following day we attempt to summit. That's the day we find out if possible truly can be redefined.