Knowing what you have been able to do in the past- The ride will be a doddle--Providing you can keep to that saturday ride and get a couple of Spin classes during the week.
Try to get a few more hills in if you can and if it were me- I would do the spin classes on Tuesday and Thurday and cut out the others. But I would work in those spin classes.
A metric is well within your capabilities. So you haven't done this much climbing before but it will not matter. Just remember that you ride hills at your pace- and no-one elses.
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How long was I in the army? Five foot seven.
Spike Milligan