I typically go along the Emerald Necklace; do a loop around the fens, run through the riverway, through Olmsted and then around Jamaica Pond. Gives me the option to extend to Arboretum/Franklin Park or keep it shorter and cut some parts out. Would 100% recommend

On occasion I’ll make my way over to the Charles and go from JFK St down to Arsenal St and then loop back around (or you could go further if you want)