I would treat the wife (If she ever came out on a ride with me) as I would a new member to our group. There would always be someone riding with them and I would only pick up any "Major" faults in their riding style. That would be the same even up hills just to make certain that they are not over extending themselves by trying to stay with us.
Admittedly in a group- it would not always be the same rider having to go slow all the time- but if you want to keep the other half riding- those first few rides will need a lot of company and encouragement.
OR-- And I honestly would not do this- Get a tandem. Then if the fit rider wants to go faster- he will soon be bought back to the other riders level.
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How long was I in the army? Five foot seven.
Spike Milligan