Perhaps a Salsa Vaya would fit your needs? It sounds like it works insofar as the mixed surfaces go and it has a more relaxed touring-style geometry. I'm not entirely sure what kind of position you want because I associate that stretched out road geometry with back pain more than a mountain bike's, so I'm thinking the kind of in-between style a Vaya would offer might be best. You also wouldn't have any problem using it to deal with errands, that's for sure.