Well I’ve done 2 days of the above, so now a fully qualified professional who’s opinion matters! (Yeah Bollox! 
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Anyway, now with my VAST experience, I’d say it’s not the driving that’s the hard part, it’s the parking when you get to your drop. I’ve been on store deliveries, and these are obviously on main streets with lots of traffic & annoying shop customers getting in the way who park exactly where you want to pull up. Kill them all I say! 
But seriously, as a newbie agency driver your brain is going ten to the dozen as you approach your drop, and you only have a split second to assess the situation and decide what to do. For a start, as you don’t know the route or store, you’ve a 50/50 chance of approaching it from the preferred direction, where the regular driver knows this in advance… Then you have the question of; is it a front or back door delivery? So you could park up in what appears to be the perfect spot, only to find you then need to get around the back somehow. Grrrrr! 
Then there is the probably fact that you have the regular driver’s full load, and while they can probably get through all the drops in their allotted time, a newbie has next to no chance of completing the drops in that time. I think I was about 30 mins behind schedule before I drove out the yard, simply because it’s all new and I didn’t want to miss anything and screw up!
But my biggest learning experience, wasn’t parking up, it was not considering my exit strategy…
So yesterday afternoon (a few hours behind schedule) I arrived at a store, and as usual there was nowhere to park out the front 
So instead of driving past and having to finding to somewhere to turn around, I spotted in the split second I had on approach, that there was a large side street what was as close to the store I was going to get. So in I pulled - Big mistake!!!
Anyway, I did the delivery, got back in the wagon and prepared to get to my next drop. However, I now had a decision to make. Do I reverse back onto the main road, which is obviously Illegal, or do something else… Having no real experience of a vehicle that size, I didn’t properly consider that it’s ■■■■ near impossible in an 18ton truck so see what’s coming on the main road, and finding a gap. Meaning that without a banksperson to help you, you’re very likely to cause an accident. So I made the decision to drive down the side street and find my way out onto the main roads that way. But what i had missed (While on my approach to the store) was the sign to say this was a dead end street. And I didn’t find that out until I was 3/4 the way down it! Bollox! So instead of having to reverse out onto a main road, I now had to reverse the wagon almost the full length of the street with about a foot of clearance on either side, plus the problem of reversing back out onto the main road. Again, Bollox!!!
So in summary, that was a very silly mistake i made and hopefully something I won’t do again. But I’m not counting on that! Clearly what i should have done, was stop on the main road and reverse into the side road. And I’m sure that’s exactly what you’re all thinking, and so am I now! 
You live and learn… 
I was sad that the traffic cleared and I had to move off… 