jimmuller 

Nickel,
re the Waltham route:

I have ridden parts of Lexington/Waltham Streets and much of the rest heading north. It should be nice though not necessarily quiet. The Hancock St part goes through some residential areas that should be pretty serene even if slightly busy on occasion.

I have not ridden the Page Rd section. The short section of rt225 between Page and the rt4/225 split will be busy but bikeable. There are easier ways to do that though. For example, you can go left on Loomis to the stoplight at S Rd (at Depot Park, across the intersection from the Bikeway Source bike shop) then go right on South back to rt225, thus missing the busy parts of Main St through Bedford Center. (More on this in a moment.)

In fact, what many people do and when we go directly from Waltham is pick up the Minuteman Bikeway in Lexington (or Arlington) and ride it out to Depot Park.
Since you are going down rt62 anyway, once at Depot park by either the Minuteman or Loomis St you can continue straight across on Railroad Ave, follow it around the school fields, take the right onto McMahon, and then left onto rt62.

Rt62 is busy but with a decently wide shoulder (except for the short newly paved section near McMahon). It carries a lot of bikes and is pretty nice for a numbered route. We do it on the tandem regularly.

We normally go left on Old Bedford Rd then right on Mass Ave/rt2A to get into Concord, but since you're going to return on part of that way you can just follow rt62. (A few blocks past where Old Bedford Rd goes left watch for the brown "Wildlife Viewing Area" & pic of binoculars sign on the left. It indicates a right onto Monsen Rd over to an observation tower by Great Meadows. There is also a restroom facility there.)

Finally, since you will be in Concord anyway you should go right on Monument (at the far end of the green) and stop at the North Bridge. You can retrace your route back to Concord Center or continue on to the first left and loop back around into Concord Center on Lowell Rd.

The return run on Virginia Rd and Mass Ave/rt2A is okay. Lots of people bike it.

One final point, there are some hills on this route, nothing super long but a few moderately steep. Enjoy the ride!