Triangle $ABC$ lies in the Cartesian Plane and has an area of 70. The coordinates of $B$ and $C$ are $(12,19)$ and $(23,20)$, respectively, and the coordinates of $A$ are $(p,q)$. The line containing the median to side $BC$ has slope $-5$. Find the largest possible value of $p+q$.
1. **Determine the midpoint \( M \) of \( \overline{BC} \):**
   Given \( B = (12, 19) \) and \( C = (23, 20) \), the coordinates of the midpoint \( M \) are:
   \[
   M = \left( \frac{12 + 23}{2}, \frac{19 + 20}{2} \right) = \left( 17.5, 19.5 \right)
   \]

2. **Find the equation of the line \( \overline{AM} \):**
   The slope of the median \( \overline{AM} \) is given as \(-5\). Using the point-slope form of the line equation:
   \[
   \frac{q - 19.5}{p - 17.5} = -5 \implies q - 19.5 = -5(p - 17.5) \implies q = -5p + 87.5 + 19.5 \implies q = -5p + 107
   \]

3. **Find the equation of the line \( \overline{BC} \):**
   The slope of \( \overline{BC} \) is:
   \[
   \text{slope of } \overline{BC} = \frac{20 - 19}{23 - 12} = \frac{1}{11}
   \]
   Using the point-slope form of the line equation with point \( B(12, 19) \):
   \[
   y - 19 = \frac{1}{11}(x - 12) \implies 11(y - 19) = x - 12 \implies 11y - x = 197
   \]
   Thus, the equation of the line \( \overline{BC} \) is:
   \[
   -x + 11y - 197 = 0
   \]

4. **Use the area of triangle \( ABC \) to find another equation:**
   The area of triangle \( ABC \) is given as 70. The formula for the area of a triangle with vertices \((x_1, y_1)\), \((x_2, y_2)\), and \((x_3, y_3)\) is:
   \[
   \text{Area} = \frac{1}{2} \left| x_1(y_2 - y_3) + x_2(y_3 - y_1) + x_3(y_1 - y_2) \right|
   \]
   Using the distance from point to line formula, the perpendicular distance from \( A(p, q) \) to the line \( -x + 11y - 197 = 0 \) is:
   \[
   \text{Distance} = \frac{| -p + 11q - 197 |}{\sqrt{(-1)^2 + 11^2}} = \frac{| -p + 11q - 197 |}{\sqrt{122}}
   \]
   The area of triangle \( ABC \) can also be expressed as:
   \[
   70 = \frac{1}{2} \times \text{base} \times \text{height} = \frac{1}{2} \times \sqrt{122} \times \frac{| -p + 11q - 197 |}{\sqrt{122}}
   \]
   Simplifying, we get:
   \[
   70 = \frac{1}{2} \times | -p + 11q - 197 | \implies 140 = | -p + 11q - 197 |
   \]
   Therefore, we have two cases:
   \[
   -p + 11q - 197 = 140 \quad \text{or} \quad -p + 11q - 197 = -140
   \]

5. **Solve the equations:**
   Substituting \( q = -5p + 107 \) into the first case:
   \[
   -p + 11(-5p + 107) - 197 = 140 \implies -p - 55p + 1177 - 197 = 140 \implies -56p + 980 = 140 \implies -56p = -840 \implies p = 15
   \]
   \[
   q = -5(15) + 107 = -75 + 107 = 32
   \]
   Therefore, \( p + q = 15 + 32 = 47 \).

   For the second case:
   \[
   -p + 11(-5p + 107) - 197 = -140 \implies -p - 55p + 1177 - 197 = -140 \implies -56p + 980 = -140 \implies -56p = -1120 \implies p = 20
   \]
   \[
   q = -5(20) + 107 = -100 + 107 = 7
   \]
   Therefore, \( p + q = 20 + 7 = 27 \).

6. **Conclusion:**
   The largest possible value of \( p + q \) is \( 47 \).

The final answer is \(\boxed{47}\).