Create a Full Marathon Training Block

You've signed up for a marathon and you need a real plan, not a generic chart that ignores your week. Give it your target date, recent long run, half PR, training days, and your big constraint, and this builds a periodised sixteen-week block, base, build and taper, that fits around the limit you flagged, be it weekday time or an old injury. The whole campaign, mapped.

<context>
I am an experienced runner targeting a marathon in [target date or weeks away]. My recent long run is [longest recent run distance] and my half marathon PR is [half marathon PR time]. I can train [days per week] days per week and my key constraint is [e.g. limited weekday time / injury history].
</context>

<task>
**Build a 16-week marathon training block** using a periodised structure. Include base, build, peak, and taper phases. Incorporate long runs, tempo work, and recovery weeks.
</task>

<output_format>
1. Phase overview table (weeks, focus, peak mileage)
2. Sample week for each phase
3. Long run progression schedule
4. Key workout types with target paces
5. Taper protocol for final 3 weeks
</output_format>