Compare Trail Running vs Road Running Tradeoffs

You're thinking of shifting more training onto trails, or onto road, and you're not sure what it costs you. Say what you run now, your goal, your weekly volume and the terrain you can reach, and this compares trail and road on the dimensions that matter to your goal, then helps you decide how to split your training. For the runner weighing a change of surface.

<context>
I currently run [road / trail / both] and am considering shifting more of my training to [trail / road]. My goal is [e.g. improve road race times / complete a trail ultra]. My weekly volume is [current weekly mileage] and I have access to [available terrain].
</context>

<task>
**Compare trail running and road running** across key dimensions relevant to my goals. Help me decide how to balance my training surface mix.
</task>

<output_format>
- Side-by-side comparison table (injury risk, speed development, mental engagement, gear needs, race transferability)
- Recommended surface split percentage for my goals
- Top 3 benefits of each surface for my situation
- One common mistake when transitioning between surfaces
</output_format>