So having topped up water and touched in with the team at the first checkpoint, I've once again taken off at speed to try and catch up the leaders.

The first people I overtake are this young couple, a blonde woman and a red-headed man – the latter of whom I've guessed is military due to his camo shorts, big leg tattoo and general air of hardness. I was wrong again: they're both students, with friends in the military – and she is in the Army Cadet Corps (like ROTC)! Hah. They ask where in the States I'm from, and I give them the flag rap, and they're both extraordinarily nice and friendly. I make my excuses and sally forth! alongside, now, a canal towpath.

So, I'm boogying on down the towpath now, keeping a death march pace, and trying to make up time. I pretty quickly realise that by far the biggest enemy of making good time is not the hills or the heat – but my own photo-shooting and notebook-scribbling, which require stopping or at least slowing to effectuate. This is dread, mon. Truly dread.

I'm caught up by the senior H4H guy who is running the marathon – after riding to the start – who slows from his jog to chat with me. I share with him my dilemma:

His answer, totally unhesitating, takes me by surprise:

And with that, he takes off at a jog again, leaving me feeling like a total wuss.

So I pick up my pace again. I try a little light jogging; walk awhile, run awhile. My feet are burning, and my inner thigh muscles are none too happy with me either. Horrifyingly, this long stretch on the towpath – which you'd think might be tree-shaded – is completely exposed to the baking sun.

I ring Tim:

I'm jogging again now, only really a few minutes after being admonished to go for it by the marathoner – when this really lovely duck appears.

And so I'm stopped there, trying to shoot the duck's arse as it dives for food, and painfully feeling all of the time I'd just made up bleeding away into nothing again. And that's when I realised: This is totally stupid. It's precisely the photography and note-taking – not to mention the bantering with nice people – that I enjoy about this whole barmy thing. Without those, it's just . . . a death march.

And so I pack in the catching-up-the-leaders mission. And, very happy with my decision, I realise I can now tarry endlessly, shooting and scribbling and bothering flora and fauna – while waiting for the others to catch me up.

My legs are definitely appreciating the new non-death-march pace. I actually fall off the back again, bothering flowers and cows.

Though, now, my hips are kind of starting to seize up a bit. I don't suppose all that squatting in the reeds doing flower portraiture helped. I see how far ahead the others have gotten (2nd and 3rd photos below). I really don't feel like I can speed up all that much. And I'm actually daunted by what it's going to take to catch up.

Now, I think, I know what it is to feel real physical limitations.

I realise I'm going to have to settle in and conserve myself.

I share with Tim my new consciousness of my limits, and my unprecedented need to not fall off the back shooting things, due to what it's going to cost me to catch up.

We stick together and talk e-books for awhile. Sarah it turns out is a bibliophile, who is no hurry to see books-as-objects go.

I really hate to say this, but my legs are definitely hating life by this point; and my feet are not terribly happy with me, either. Still trying to be jaunty, I tell Tim, "Well, I know what I'm going to be doing at the health club all next week: swimming."

We start a fearsomely long straight stretch, mostly uphill.

I think everyone's hurting a bit, though everyone's afraid to say it out loud. I mean, we're not even halfway!

And I know this is pretty cheesy, but maybe also kind of appropriate, but I find myself actually thinking about today's challenge in the context of the challenges routinely borne by the boys and girls on whose behalf we're doing this. I think of infantry routinely doing 20-mile tabs with 80lb of kit on, body armour, helmets, weapons and ammunition, and all in the 105° Afghan desert summer.

Plus being shot at. And IED'd.

Due to knackeredness, I'm kind of just thrusting the camera out toward pretty scenes and hoping the pictures come out. I realise it hasn't even occurred to me to look at how much memory I've got left, and I further realise – and tell Tim – that we've crossed a real technology threshold: effectively, infinite film.

Smart aleck. ;^)

Finally, lunch blessed lunch. We plop down in the dirt, where just the sitting is delicious, and guzzle water and cold, 100% pure orange juice, and stick food in our faces. And try not to think too much about the fact that we're exactly at the halfway point.