I would run 1x for the maratona as well. But that's me. The first step is to think about what biggest gear you REALLY need. do you need 42x9? That's like a 52x11. I know that's not a "strange" gear to have on a road bike, but do you really need it? That's 2% smaller than what Froome, Martin, Cancellara and Sagan ride their road races with, is anybody who pays for their bikes really that good? Not on their own power I think, and frankly not with help from gravity on the descents either.

So my first step is always to encourage people to be honest about the biggest gear they really need. And if you spin out once, is that that bad? Is it worth the bigger gaps in the gearing, is it worth the weight and complexity of a 2x setup? Or look at it this way, on your current set-up, how often are you really in the smallest cog? Or even the second-smallest. Would it really hurt if you missed those? If not, figure out what gear it is that you really need, and start from there.

Of course you need to then calculate the ratio, not the actual number of teeth, since you will still want to start your cassette with a 9, 10 or 11 (depending on the cassette you choose). So if your current road bike has a 52/36 crank and an 11-28 cassette and you rarely use the 11 & 12, then 52x13 is really the biggest gear you need, or a ratio of 52/13 = 4.00.

Then do the same on the other side. Is 36x28 enough as you have it now, or would you like 10% smaller or 20% smaller or ...? Say you want 10% smaller, that means you would like 36/28=1.29 and 10% smaller is 1.16. Then you start the math. If you would have a "regular cassette" starting with an 11, then you need a 44t front ring (11x4.00) and the biggest cog you would need is a 44/1.16=38t. And 11-38 cassette is tough to find, but there are a lot of 11-36 cassettes (if you think you don't really quite need 10% smaller than you have, and 4% smaller is enough) or you can go with a Shimano MTB cassette with 11-40 (if you decide that 16% smaller gear than you have now on the 2x is a better option).

If you do the above math and you can't get the range you need starting with the 11 (although the new Shimano 11-46, there are broad options), you can try if the 10-42 is going to work and if that doesn't, then the 9-44 will do the trick).

I wouldn't mind the gaps even on the 9-44, but if you don't really need all that range (after being honest about the biggest gear), then smaller steps are of course better.

Hope that helps.

That's true, the Nano 40c is a "really big 40mm tire", I know they should all be the same but most 40mm tires are quite a bit smaller. I ride it a lot though, hasn't been a problem. I also ride the Exposure, but the 30mm (and I have 34mm Exposures on my "city bike".