Fischer has him riding down Battle Road the day after April 19th.
FWIW, here's the segment of Adams' diary which deals with this event. Sure wish he'd said more...
QuoteA few days after this Event [April 19th] I rode to Cambridge where I saw General Ward, General Heath, General Joseph Warren, and the New England Army. There was great Confusion and much distress: Artillery, Arms, Cloathing were wanting and a sufficient Supply of Provisions not easily obtained. Neither the officers nor Men however wanted Spirits or Resolution. I rode from thence to Lexington and along the Scene of Action for many miles and enquired of the Inhabitants, the Circumstances. These were not calculated to diminish my Ardour in the Cause. They on the Contrary convinced me that the Die was cast, the Rubicon passed, and as Lord Mansfield expressed it in Parliament, if We did not defend ourselves they would kill Us.
Source: http://www.masshist.org/digitaladams/aea/cfm/doc.cfm?id=A1_18
Fischer captures the spirit of this quite well, I think.