Cutting through Roslindale can at least get you off of a portion of Hyde Park Ave and skip the Forest Hills traffic- left on Blakemore, right on Florence, right on Firth, and left onto the Washington St bike lanes to the entrance to the lower MBTA parking lot, through the tunnel continue one block then right on Fairview and into the Arnold Arboretum, left on the path over Peter's Hill and work your way through the paths to the main Arboretum gate near Murray Circle (Arborway and Centre Street) Cross the Arborway and follow the parallel street until you can cross the Jamaicaway to get onto the Emerald Necklace bike path and head through Brookline to Washington St.
Just remember to ride slowly through the Arboretum and look out for pedestrians and poodles.
Brookline is looking at improving pedestrian and bicycle crossings along their portion of the Emerald Necklace, especially at rte 9: www.brooklinema.gov/EmeraldProject Their next meeting is April 4th at 6pm in Brookline Town Hall.